The present invention relates to a device for tensioning knitted fabrics in knitting machines, particularly in large-diameter circular machines.
It is known that machines for producing knitted fabrics can be classified in three broad categories: straight-bar machines, circular machines for producing a closed tubular fabric, and circular machines for producing an open tubular fabric.
Straight-bar machines use devices for tensioning the fabric being formed which are generally constituted by two or three rollers or by pairs of mutually facing narrow belts which are arranged below the needle work area. The fabric formed by the needles passes between the rollers or narrow belts, which can be actuated so as to apply to the fabric being formed the chosen degree of tensioning, which is necessary to facilitate the needle in holding the stitch.
Circular machines for manufacturing a closed tubular fabric usually have tensioning devices which are accommodated inside the needle cylinder of the machine and have adapted elements which grip the fabric that descends inside the needle cylinder and is gradually wound onto a specifically provided roll located in the footing of the machine. The fabric gripping elements vary as a function of the type of machine but usually apply grip to the entire fabric.
In circular machines for producing an open tubular fabric, generally large-diameter circular machines, there is a plurality of tensioning rollers which are arranged so that their axes lie along a polygonal line inside the needle cylinder and below the needle work area. The tensioning rollers face respective contrast rollers which are usually supported so as to freely rotate about their axes and are parallel to the axis of the correspond tensioning roller. The tensioning rollers are actuated so as to rotate about their respective axes in order to tension the fabric that passes between the tensioning rollers and the contrast rollers. The tensioning rollers are actuated by means of a single motor, with an interposed single reduction unit for all of the rollers, which is torque-controlled and can remain tensioned even when no fabric is being formed.
In any case, when knitting machines start to produce fabric, they do so by gradually knitting a plurality of stitch sets along the transverse extension of the fabric. When this occurs, the sets that produce knitting loosen the tension at the needles that are already knitting, while the situation remains unchanged on the remaining part of the needles. The force applied to the fabric by conventional tensioning devices is thus discharged onto a number of needles which gradually decreases; accordingly, the specific tension on said number of needles is increased. Because of this fact, tension is unequal in the various regions of the fabric, and this is a problem since it can cause knitting defects which are all the more evident as the difference between the length of the fabric formed by a given group of needles and the length of the fabric formed by other needle groups increases.
This problem is even greater in case of particular knitting styles, such as for example braiding, which are particularly appreciated from an aesthetic point of view and are characterized by extreme differences in style among the regions of the same fabric sheet. The different degree of tension of the fabric in the various regions in fact leads to different stitch-holding conditions of the various needles, with the possibility of knitting defects if this difference is particularly high, or in any case to uneven stitch-holding conditions which negatively affect the quality of the fabric.